feat: Allow to install default rust packages

[Wihrt]

The purpose of this Pull Request is to mimic the behavior of python or nodejs asdf plugin, and to allow to install default rust packages. Each package will be installed inside the rust folder.

[brennanfee]

Is this repo, and by extension this PR dead?

[Wihrt]

For the repo I cannot say. For the PR, I need to fix some things, but the main feature is present. Hopefully, I will take a look this week ;)

[RobLoach]

~/.default-rust-packages doesn't really seem standard. Wouldn't you want this as a Cargo.toml instead?

[Wihrt]

Nor the different default-<whatever>-packages used to install packages ;) I could handle both ways (1 package per line and TOML), but honestly, if this PR is merged, it would be already great ;)

The handling of the TOML format seems a 'Nice to have' feature, IMHO.

[Wihrt]

@smorimoto For you review ;)